This invention relates to apparatus for starting and operating a high intensity discharge lamp and, more particularly, to a voltage multiplier starting circuit energized by a high frequency AC voltage source and operative to produce high voltage, high frequency ignition pulses for initiating an arc discharge in a HID lamp, in particular a miniature metal halide lamp.
HID lamps, and especially metal halide lamps, have rather stringent requirements as to the starting voltage, the reignition voltage and the lamp current waveform. Prior art ignition circuits for HID lamps are designed for use with a 60 Hz line source. Ignition pulses are delivered at a maximum rate of one for each half cycle or at most a 120 Hz repetition rate. The use of a high frequency source such as an electronic inverter will result in a smaller, lower loss ballast. It also makes it possible to deliver ignition pulses at a higher repetition rate, which gives improved lamp ignition and/or ignition at a lower peak voltage.
As the repetition rate is increased, it is important that the losses in the starting circuit and the loading on the inverter be limited to a safe value so that if lamp ignition does not take place (such as at the end of lamp life), circuit failure will not occur.
One of the first circuits developed to generate high voltage ignition pulses for starting metal vapor or high pressure sodium discharge lamps of the type that require relatively high ignition voltage pulses in order to provide reliable ignition thereof is shown in U.S. Pat. No. 3,917,976 (11/4/75). This prior art circuit operates directly from a 120V, 60 Hz AC supply voltage.
The invention also relates to a switching arrangement for starting a high-pressure discharge lamp provided with a first supply source connection point for connecting a supply source and with at least one lamp connection point for connection to the high-pressure discharge lamp. An electric coil with a tap point is connected between said supply source connection point and said lamp connection point. The switching arrangement further comprises a first and a second branch each including a diode and each connected to the coil. One of the branches is connected to the tap point on the coil. Both diodes are connected to each other by a third branch which includes a semiconductor switch. The third branch is connected at one side directly both to the coil and the diode of the second branch and the first branch includes a first capacitor coupled between the coil and the third branch and the relevant diode. The first and second branches are connected through a common impedance comprising a second capacitor to a second supply source connection point.
This kind of switching arrangement is described in U.S. Pat. No. 4,337,417 (6/19/82). In this known switching arrangement, the common impedance includes a resistor of substantial resistance value. The resistor will influence the rate of charge of the second capacitor and the resistor also insures that the voltage pulse produced in the switching arrangement does not flow away directly to the supply source This requires that the resistor have a high value. The resistor then substantially reduces the voltage pulse repetition frequency due to its high value. This is especially important where the supply source has a high frequency, at least a frequency which is considerably higher than 50 Hz.
The above patent uses a voltage doubler as part of the starting circuit. It is energized directly from the 60 Hz, 120 V AC supply and uses a transformer with a step-up transformation ratio of 20:1. The resistor in the charge path of the capacitors of the voltage doubler limits its use to low pulse repetition rates. High frequency operation with a 10 KHz AC source, even with only 1-2 KHz ignition pulse repetition rates, would result in excessive losses in the resistor or in loading of the pulses by the ignition circuit.
Another circuit which features a voltage doubler starting circuit for sodium vapor street lamps is U.S. Pat. No. 4,209,730 (6/24/80). High voltage ignition pulses are periodically applied to the lamp via the ballast. This circuit also operates directly from the 60 Hz, AC supply voltage.
U.S. Pat. No. 4,143,304 (3/6/79) also utilizes a voltage doubler circuit for starting a high pressure sodium discharge lamp and operates directly from the 60 Hz AC supply line. It uses a charge resistor with the attendant disadvantages of lower efficiency etc. mentioned above
The prior art circuits described above are adequate for the ignition and operation of high-pressure sodium discharge lamps from a 60 Hz AC supply, but are unsuitable for the ignition and operation of HID lamps, especially miniature metal halide lamps, at high frequencies such as 10 KHz and above.